There are a number of materials used to make vests ranging from kevlar to polycarbonate and titanium. It really depends on your personal preference. Personally I like the feel and protection that a hard stab vest offers, especially against blunt trauma. Companies like Protect Yourself who sell these hard stab vests state that they are more effective against a wider range of threats than normal vests. If you require bullet protection a dual vest is the way to go. Price wise a good stab proof vest can cost anywhere from £190 - £500 depending on the protection level. I do often advice not to buy second hand body armour, though technically some protection is better than none, if it fails you can die. Body armour is a small investment to protect life.
When buying a vest on-line the best advice is to research the supplier, make sure they know what they are talking about, have a chat with them. Have a good idea of the protection you need, get a few prices, and take some time to make a decision. The armour you buy will serve you for many years, look for quality, as well as cost.